Nato is pressing ahead with expansion at its Prague summit, where it has invited a group of Baltic and Eastern European nations to join the alliance. BBC News Online World Affairs correspondent Paul Reynolds examines the issues.

Which countries are going to join?

Seven countries have been invited to join the alliance at the Prague summit: the three Baltic republics of Estonia, Latvia and Lithuania; Slovenia; Slovakia; Bulgaria and Romania.

Two other applicants - Albania and the Former Yugoslav Republic of Macedonia - are likely to be told they have not met the criteria (economic, political and military reform) and will have to wait. Croatia applied only this year and has just started the process.

Why is Nato expanding?

For political and security reasons. The idea is to encourage the growth of democracy and market economies in the former Communist states of central and eastern Europe and to lock these in with Nato (and also European Union) membership.

But the new members themselves also feel more secure by being within the Nato shield. Some of them are still fearful of Russia. Nato expanded from 16 to 19 when the Czech Republic, Hungary and Poland joined in 1999.

Has Russia dropped its objections to expansion?

It is still not happy but it has had to accept the inevitable. Boris Yeltsin once tried to get Bill Clinton to give a "gentleman's agreement" that the Baltic republics would not be allowed to join but no such undertaking was given.

President Putin has concentrated more on raising specific concerns than objecting in principle. These concerns include the forward deployment of Nato troops to the Russian border - which won't happen.

Would the new members have to be defended, even with nuclear weapons?

Yes, in theory, because Nato is an alliance. An attack on one is seen as an attack on all. But a decision has to be taken in practice if there is an attack. The likelihood of that happening is remote given the new relationship with Russia.

What would the new countries contribute to Nato?

Not much in the way of military muscle. But Nato is changing from a defensive alliance ready to repel invasion to a more flexible provider of forces for crises worldwide. Individual countries are being encouraged to specialise. The new members might be able to take part in that process. Their main value, however, is that they project Nato's political weight further east, up to the Russian border and down to the Black Sea.

Isn't their kit old Soviet stuff - can their radios talk to ours?

Yes, a lot of it is old stuff and no, their radios don't talk to ours and anyway what language would they use? But they are not really joining to add to Nato's armoury. They will modernise their forces over time.